### LeetCode Challenge #142. Linked List Cycle II

Given the `head` of a linked list, return the node where the cycle begins. If there is no cycle, return `null`.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the `next` pointer. Internally, `pos` is used to denote the index of the node that tail’s `next` pointer is connected to (0-indexed). It is `-1` if there is no cycle. Note that `pos` is not passed as a parameter.

Do not modify the linked list.

Example 1:

```Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.
```

Example 2:

```Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.
```

Example 3:

```Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.
```

Constraints:

• The number of the nodes in the list is in the range `[0, 104]`.
• `-105 <= Node.val <= 105`
• `pos` is `-1` or a valid index in the linked-list.

Follow up: Can you solve it using `O(1)` (i.e. constant) memory?

##### Video Solution
###### Java Solution
```				```
public class Solution {
public ListNode detectCycle(ListNode head) {
if(head == null){
return null;
}

ListNode fast = head ;
ListNode slow = head ;
ListNode ptr = head ;
while(fast!=null && fast.next!=null){
fast = fast.next.next;
slow = slow.next;

if(slow==fast){ // there is a cycle in ll

while(ptr!=slow){
ptr = ptr.next;
slow = slow.next;
}

return ptr ;
}
}

return null;
}
}
```
```